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My ears are burning Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 30 Dec 2008 15:35:25
In Reply to: Charging issue on a 95 CSE turbo, GaryJ, Tue, 30 Dec 2008 12:34:34 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Need some basics about the original problem.
You turn the key to ON, but you don't get a battery icon. I assume you do get an oil light, ABS, etc. Just checking.
When you say "If I start the car and let it idle it will not charge." I assume you mean the voltage, as read by the EDU, stays down in the low 12.X range, which isn't charging. If you rev the engine, it pops up into the 13.X range, which is charging.
You are quite right about the light - it is integral to proper alternator operation. An alternator can be thought of as a generator, but the magnetic field in an alternator is created by an electromagnet, not permanent magnets. If that electromagnet (field coil) has no voltage, there is no magnetic field, and the alternator is just so much spinning wire and metal.
When you turn the key to ON, voltage flows from the battery, through the key, through the Battery bulb in the dash, and through a thin wire (usually yellow or green) to the D+ post on the alternator. This is connected to the field coil and then to ground. That current is enough to power the field coil. The light turns on because current flow through the filament. When the engine starts and the alternator starts spinning, it can make its own electricity; the voltage regulator then starts putting voltage on the D+ part of the coil. Since the D+ is now near 12 volts, as is the other side of the BATTERY bulb, no current flows through the bulb and the light goes out.
It appears that in your case, not enough current is flowing through the bulb to light it properly, and to charge the Field Coil. But some current is - when you rev the engine, the alternator spins faster, and can produce just enough voltage in the weak magnetic field for the VR to take over, and all is good.
I strongly suspect a bad alternator ground, but a broken bulb circuit is also in the running. But first things first. Find the thin green (or yellow) wire going to the D+ post on the alternator. Take it off the alternator and touch it to ground, while the key is in the ON position. The BATTERY light should light. If it does not, you need to troubleshoot that. The bulb may be bad, or just poorly seated. The wire may have broken (unlikely), but I have seen the wire get pretty corroded.
If the light comes ON when you touch the wire to ground, then I'd suspect a bad alternator ground. If you've got a good volt-ohm meter, you can measure the resistance - it should be well under an ohm. However, most ohm meters can't tell the difference between 2 ohms and a dead short. With the wire connected back to the D+ terminal, put one lead of the meter, set to VOLTS, on the negative terminal of the battery. Turn the key to ON, but don't start the car. Touch the other meter lead to the alternator body. It should be within 0.1 volts of 0 -basically, at the same voltage as the negative battery terminal. If it isn't, say 2, 3 or 12 volts, then the alternator ground is bad. If it is within 0.1 volts of 0, touch the meter to the D+ terminal. If it reads 12 volts, then something is bad internal to the alternator - it should be a low voltage - between 0 and 4 volts or so.
Why measure volts instead of ohms? Push current through a resistance, and you get volts. It's hard to measure 0.1 ohms, but put an amp of current through it, and you'll read 0.1 volts, easily done by even a cheap meter.
Yes, I know, touching leads to the back of the alternator requires a gymnast with ET's fingers. Sigh.
I suspect the bad alternator ground because the alternator is getting some current from somewhere to charge the field coils. It's possible enough is getting through the bulb, just not enough to light it. There would be other charging issues. A bad bulb circuit is most probable, simply because it has the most things to go wrong.
posted by 76.221.218...
, Tue, 30 Dec 2008 15:35:25 <-- Viewing This Message, Wed, 31 Dec 2008 13:52:08
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